Move pathExists() to Utils.cpp

This is useful as a general utility function.

Change-Id: Id43fc106dc6c544c6e4ce65f10c7d4246b99e54a
diff --git a/Utils.h b/Utils.h
index a727e63..cf85136 100644
--- a/Utils.h
+++ b/Utils.h
@@ -179,6 +179,8 @@
 
 status_t WaitForFile(const char* filename, std::chrono::nanoseconds timeout);
 
+bool pathExists(const std::string& path);
+
 bool FsyncDirectory(const std::string& dirname);
 
 bool writeStringToFile(const std::string& payload, const std::string& filename);